To explain to you why so.
This is reported speech. Confusing when you learn English-Ukrainian/Russian
In English
Anna said: "I will learn Ukrainian". = Anna said that she would learn Ukrainian
In Ukrainian when making such sentences we do not change the time at the reported part of the sentence
Анна сказала: "Я буду вчити Українську". = Анна сказала, що (вона) буде вчити Українську